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Part IV

Troubleshooting and Repairing ATX

Power Supply

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19) Troubleshooting and Repairing Computer ATX Power Supplies

Introduction

Computer power supply unit or PSU is the device that converts the input AC voltage to the DC voltage needed by the personal computer. ATX stands for Advanced Technology Extended. Not like the old AT PSU, an ATX power supply does not directly connect to the computer system power button, it allows the computer to be turned off via software. However, many ATX power supplies have a manual switch on the back to make sure that the computer is truly off and no power is being sent to the components. If this switch is on, energy will flows to the components even when the computer appears to be "off" unless you press the on button. This is known as soft-off or standby. Since the introduction of IBM PC/XT there has been about a dozen different Power Supplies types in the market such as AT, Baby AT, ATX, BTX, LPX, SFX, EPS, WTX, EBX, TFX, LFX, CFX. They differ by their structure, size, form factors, volt/amp ratings and connectors. While they may look different on the outside, most PC power supplies use the same electronics on the inside and it�s not difficult to fix them. In this chapter I�m going to explain only about the ATX power supplies since they are most commonly in used today. Having a higher wattage power supply than the required capacity is always desirable, as you can add new devices like hard drives, CD/DVD drives, tape drives, ventilation fans, etc to the computer without worrying about whether the power supply can provide enough power. If your requirement is 300 watt, then fit your computer with a 350 watt power supply. Additional wattage does not mean that your electricity bill will be higher. It just gives you an option to add additional devices to your computer and also there will be lesser load on the power supply. A 350 watt power supply will consume only 200 watt of power if the devices connected to it consume 200 watt of work. By using a PSU that delivers more power than required, it means it won't be running at full capacity; which can prolong life by reducing heat damage to the PSU's internal components during long periods of use. Always replace a power supply with an equivalent or superior power output (Wattage).

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Generally ATX power supply can be broken down into two types. The first type would be the one that that use the Mosfet technology (figure 19.1) with Power IC and Power FET to drive the transformer. The second type would be the one that use the Half bridge topology (figure 19.2) where a pair of transistors was used to switch the high voltage supplies across the primary winding of the SMPS transformer. I would only explain the second type as the first type explanation could be found in chapter 3.

Figure 19.1- ATX SMPS That Use The Power FET And Power IC

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Figure 19.2- A Half Bridge Topology ATX SMPS

Figure 19.3- Block Diagram Of A Half Bridge Topology SMPS

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How The Half Bridge Topology SMPS Works The main AC supply first enters a rectifier/filter circuit, which converts the AC main supply into a very high value DC voltage and filter it. This high voltage DC supply is then given to a switching transistor circuit. The switching transistors are switched on and off at a very high speed by a control circuit which produces very high frequency square wave pulses. The switching transistors switches the given high voltage DC, on and off at the same high frequency and gives square wave pulses as the output. These square wave pulses are then given to the primary winding of Switch Mode Transformer. These pulses induce a voltage at the primary winding of the transformer which will generate voltages at the secondary windings. The voltages at the secondary winding is then rectified and filtered to produce the required output. In order to regulate the output, one of the generated output voltages is sent back to the switching section. This voltage is first sent to a sense amplifier circuit which compares it with a reference voltage and generates an error voltage. This error voltage is then given to the control circuit which controls the switching transistors to regulate the output voltage. The function of the error voltage is such that if there is an increase in the output voltage, the error voltage will reduce the �On� time of the switching transistors, which reduces the output voltages. When there is a drop in the output voltage, the error voltage increases the �On� time of the switching transistors, which increases the output voltage. By controlling the �On� time, this would make the output stable in both the high or low output conditions. Other than this basic operation, most of the SMPS has the capabilities to protect itself from overload and short circuit in the output section.

ATX Power Supply Connector Pinouts And Signals

Generally there are two types of ATX power supplies currently in the market and with different type of connectors. 1) ATX version 1 Power Supply 2) ATX version 2 Power Supply

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Having an understanding of both types of ATX power supplies connector pinouts and signals could help you to turn �On� the power supply and also what voltage to expect from the output for testing and repairing purposes. For your information, obviously there are more latest PC power supplies cables and connectors coming out to the market every now and then but I fell that if you understand these two types of ATX cables and connectors, you would be able to perform the testing too in newer type of power supplies that have newer type of cable and connector configuration.

Pin out configuration of ATX version 1 Power Supply

Figure 19.4- An ATX Version 1 Connector

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Figure 19.5- 20 pin Molex ATX Version 1 Power Supply Connector Pin Signal Wire

Color Description

1 3.3V Orange +3.3 VDC 2 3.3V Orange +3.3 VDC 3 COM Black Ground 4 5V Red +5 VDC 5 COM Black Ground 6 5V Red +5 VDC 7 COM Black Ground 8 PWR_OK Gray Power OK is a status signal generated by

the power supply to notify the computer that the DC operating voltages are within the ranges required for proper computer operation (+5 VDC when power is OK)

9 5VSB Purple +5 V Standby Voltage 10 12V Yellow +12 VDC 11 3.3V Orange +3.3 VDC (Brown is +3.3 VDC Sense) 12 -12V Blue -12 VDC 13 COM Black Ground 14 PS_ON Green Power Supply On (Active Low). Short

this Green wire to ground to switch power supply On. Disconnect from

ground to switch off 15 COM Black Ground 16 COM Black Ground 17 COM Black Ground 18 -5V White -5 VDC 19 5V Red +5 VDC 20 5V Red +5 VDC

Figure 19.6- ATX Version 1 Power Supply Pin-outs

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Some ATX version 1 power supplies have extra connectors as seen in the photo below

Figure 19.7- Types Of Connectors

The 4 Pin Molex P4 12V Power Connector is use specifically for Pentium 4 Processor Motherboards while the 6 Pin AUX power connector was added to provide extra wattage to motherboards for 3.3 and 5 volts. This connector is rarely used anymore. It's most commonly found on older dual CPU AMD motherboards.

Pin out configuration of ATX version 2 Power Supply Changes to ATX standard were made to support 75 watt PCI Express cards requirements. New ATX version 2 uses new connector with an extra +3.3 V, +5 V, +12V and ground. Most motherboards nowadays allow using an old ATX version 1 power supply with 20 pins connector that can fits in a 24 pin socket.

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Figure 19.8- 24 pin Molex ATX Version 2 Power Supply Connector

Figure 19.9- An ATX Version 2 Power Supply Connector Pin Signal Wire

Color Description

1 3.3V Orange +3.3 VDC 2 3.3V Orange +3.3 VDC 3 COM Black Ground 4 5V Red +5 VDC 5 COM Black Ground 6 5V Red +5 VDC 7 COM Black Ground 8 PWR_OK Gray Power OK is a status signal generated by

the power supply to notify the computer

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that the DC operating voltages are within the ranges required for proper computer operation (+5 VDC when power is OK)

9 5VSB Purple +5 V Standby Voltage 10 12V Yellow +12 VDC 11 12V Yellow +12VDC 12 3.3V Orange +3.3VDC 13 3.3V Orange +3.3VDC 14 -12V Blue -12VDC 15 COM Black Ground 16 PS_ON Green Power Supply On (Active Low). Short

this Green wire to ground to switch power supply On. Disconnect from

ground to switch off 17 COM Black Ground 18 COM Black Ground 19 COM Black Ground 20 -5V White -5 VDC 21 5V Red +5VDC 22 5V Red +5VDC 23 5V Red +5VDC 24 COM Black Ground

Figure 19.10- ATX Version 2 Power Supply Pin-outs

Note: There is a curiosity in the new version of the ATX version 2 Pinouts specification. It is about the pin 20 (-5 volt) connection because in certain specification this pin is no longer in used and marked as NC (Not connected). However, according to the manuals of certain motherboards with a new 24 pin connector, the -5 volt is still present. Due to this, you must keep in mind that when you want to test a power supply with a 24 pin connector, the -5 volt output may or may not exist. The -5 volt should always be present on a 20 pin connector. The PWR_OK output (pin 8) of ATX version 1 and 2, also called PWR_GOOD or PWR_ON, is used by the power supply to show that the most important outputs (+12 V, +5 V and +3.3 V) are within their limits and can supply a nominal current.

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Troubleshooting and Repairing ATX SMPS

When it comes to troubleshooting and repairing any kind of ATX power supplies, the most important thing that you need to do is to make sure that the power supply is connected to a sufficient load like to a motherboard and hard disk (you can buy cheap and used motherboard and hard disk from any local computer shop) as seen from figure 19.11.

Fig 19.11- Connect The ATX Power Supply To A Motherboard And A Harddisk

Fewer loads will cause the power supply not to start and you may have thought that the power supply is not working. Time will be wasted if you troubleshoot a working power supply unit due to that you do not know how to properly test it with a load. Remember, not to test power supplies without load as many ATX power supplies in the market require a load for it to work properly. If you search the Internet for this keyword �ATX Power Supply Tester� you will get some results of websites selling the ATX power supply tester. Figure 19.12 in the next page shows one of the tester and an optional load to substitute the motherboard.

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Figure 19.12- Coolmax ATX Power Supply Tester Note: In certain types of power supply design, the power supply would not start up until you have connected the power supply to the original load which is the computer system itself. If you connect to other types of motherboard (although has the same connector) the power supply either totally cannot start or the fan would turn a little bit and stop. So you must test the particular power supply with the right load. Next, you must know which one is the PS_On pin (Green wire) of the power supply. In order to power up the PSU for testing, you need to short PS_ON pin with one of the common pins (ground pin). Normally, PS_ON is activated when you press and release the computer power button while it is in standby mode.

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Figure 19.13- This Is How You Connect A Wire Between The Green Wire And Ground

A lot of PC power supplies failures are actually simple problems that are easy to fix. The common problems of power supplies are failures in the filter capacitors (bad ESR or bulged) and shorted output diodes in the secondary side. Dry joints could also cause the power supply to stop working or causing intermittent problem. Before you begin to repair any power supply, you must understand about the safety precaution first. If you don�t know what you are doing or not confident about the repair, then please stop now until you have someone to guide you by your side. Please make sure that you know how the power supply works and how to accurately test the electronic components in the power supplies for higher chances of repair rate. However, if the power supply has too many burnt components in it then replacement with a new unit is the best solution.

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Problems In ATX Computer Power Supply Switch mode power supplies aren�t typically too hard to repair. They tend to fall into some categories: 1) Dead And Silent With Fuse Blown Once you have opened up the casing and found that the fuse has blown, you have to expect some serious problem in the power supply unit. Of course there are also chances that the fuse has blown by itself due to life span of the fuse or from a mild surge. You may need to use the light bulb method as explained in chapter 13 to see if the problem is only the fuse or it could be some other components� fault. From experience, if you have discovered that the fuse blown, please check on these components like Varistor, Bridge rectifier, Big filter capacitor (swollen, leaked, open or short circuit), switching transistor, power IC and secondary output diodes (Schottky diodes) for short circuit. If you find shorted components anywhere in the primary side, you should also check the resistors for open circuit and replace as necessary. Do replace the bad components; fix any cracked solder joints, reassemble and then you are ready to test it. Use the light bulb method as explained in chapter 13 before you begin to put in the fuse and turn it On. Sometimes a severed power surge could cause the Varistor and the Thermistor to split open. If you found that the primary side components burnt beyond recognition, (quite common if the SMPS was hit by lightning and had a major short circuit problem) you may need to refer to chapter 16 for further action. 2) Dead And Silent With Fuse Good If the Main Fuse is Okay then I suggest that you check for an open start up resistor in the primary side (common problem) and also for a leaky switching transistor. Directly replace the power IC as we can�t accurately test the power IC with ohmmeter. A direct short circuit of the secondary output diodes could also cause no power symptom. 3) Power Chirping With Fuse Good Chirping supplies (you could hear the sound of chirp-chirp-chirp) generally means there is a problem with the output. Shorted secondary

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output diodes (Ultra fast or Schottky diodes) are the main cause of power supply chirping. 4) Power Cycling By placing the test probe at the output connector, you will notice that the voltage reading goes up and down. The power supply is in a repeating cycle attempts to start up, but being dragged down by the overload and thus shut down and then the cycle repeats. Since you could measure the output (although the output voltage is up and down) this suggests that the primary side switching transistor is working. If it is not working there would be no output at all. Thus your repair concentration should be in the secondary side. Check on the secondary output diodes and filter capacitors. If the SMPS used power IC in the primary side, try checking the corresponding components like diode, current sense resistor or even the electrolytic capacitors before you replace the power IC and retest the power supply. If the SMPS used a Power IC, generally there would be a feedback circuit that uses the Optoisolator IC for regulation purposes. Check all the components in this area or even directly replace the Optoisolator IC and TL431 IC and retest the power supply.

Figure 19.14- SMPS Feedback Circuit

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Note: Many computer SMPS require a minimum load to maintain stability and to provide proper regulation, thus when there is not enough load (under loaded), the power supply may be cycling due to overvoltage. Make sure that the load is enough otherwise you may be troubleshooting on a working SMPS and this would waste your precious repair time. 5) Fan Turns A Little Bit And Then Stops

Figure 19.15- The Fan Turns For A While And Then Stops This suggests that the power supply is trying to start but because of faulty components in it, the power supply shuts itself off. In this case most probably the problem is in the secondary side. Bad ESR in secondary filter capacitor and capacitor bulged are very common in causing this kind of problem. These capacitors are specially designed to handle the rigors of filtering in a switch mode power supply and make sure that you test all these filter capacitors with an ESR meter. Any capacitor that is swollen or leaking should be replaced. Faulty PWM IC (i.e.; TL494CN IC and etc) and corresponding components like resistor turned into high ohm could cause similar problem too. Don�t overlook that the regulation and the feedback circuit components which includes the Optoisolator IC (if the circuit has one) may also contribute to such problem. Check that the power supply is connected to a load (or original load) if not, the power supply fan would turn for a while and then stop.

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6) High Pitched or Hissing Sound Problem-The power supply was working good (with good outputs) but it produced an annoying high pitched sound. If the power supply is working good and you hear the high pitched sound then the problem could be in the SMPS transformer or from the secondary side inductor. If the transformer or inductor was not wound tightly and secured, they would vibrate. Try gently knocking on the transformer and the secondary side inductor with the handle of a screw driver while the power supply is running. If the sound goes away then I guess you have to remove either the transformer or the inductor (whichever one that produces the sound) and dip it into shellac and let it dry for a day. This method will usually get rid of the high pitched sound.

Components Replacement If possible, get back the same part number to avoid repeating failures in SMPS that you have repaired and also to maintain the specifications within acceptable limits with respect to line isolation and to minimize fire hazards. However, if you still could not get the exact replacement part, then the only option you have is to get the nearest possible replacement part for substitution. Please refer to chapter 6 on how to get a replacement part number to insure reliable operation of the SMPS.

External Voltage Test Method

Figure 19.16- A TL494CN PWM IC In SMPS

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If you come across any power supply that uses a PWM IC in the secondary side i.e. TL494CN then you can perform the following test to make sure it is good because if it doesn�t work, there will be no power. The PWM TL494 IC is the one that drive the switching transistors in the power supply. Although we don�t use oscilloscope in every SMPS repair, without it we may have some disadvantages especially in checking for the right signal or waveform. The problem with SMPS is that if the SMPS would not work, then how do we use the oscilloscope to test out the waveform? The solution is to use an external voltage source to power up the PWM TL494CN IC and check for the waveform. First, check from the data sheet to see what is the DC voltage to power up the IC. Your ultimate aim is to supply the IC with the right voltage so that you could measure the waveform with your oscilloscope. Now, connect a 12 volt dc supply to the input pin (pin 12) of the IC and the negative to pin 7 (ground pin) or to the secondary cold ground. If you do not have a DC power supply, you may use batteries to power up the IC- it works too! Once the 12 volt is applied to the IC, use your scope to check the output waveform at pin 8 and 11. You should expect an active square waves as seen in figure 19.17 in the next page. If there is no output, try ground pin 4 (dead time control pin) and check again the output. If there is still no output, do replace the IC and retest. Do not overlook that sometimes a shorted or bad components that correspond to the IC could cause no output. For example, a bad LM339 comparator IC or an open resistor in the corresponding circuit could cause no output from the PWM TL494CN IC.

Figure 19.17- A Typical Squarewave Waveform At Pin 8 and 11 Of PWM IC TL494CN

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This method will eliminate problem to someone who is afraid of doing troubleshooting while the SMPS is �On�. They are afraid of dealing with the high voltage thus this method would suit them the most. Note: You can�t use this method to test on all kinds of PWM IC because most of the PWM IC has a feedback pin. That means if by just applying only DC voltage to the PWM IC without any signal to the feedback pin, no waveform will be produced. You have to make your own test from the information that you get from the IC datasheet and see the result for yourself because there are too many PWM ICs in the market.

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20) ATX Power Supply Repair True Case Histories

1) No power Case no 1: Most electronic repairer knows that whenever there was a complaint of no power by customer, the first thing he or she will do is to check the fuse first with an ohm meter or with a continuity test. In this case, the main fuse was found to be bad and I did not stop there. I continued to look for any burnt components, bulged capacitors, loose joints and etc. I have also tested on some of the major components like bridge rectifier, switching transistor, Varistor (A shorted Varistor tends to blow up the main fuse) and secondary output diodes. Since all other components were tested good, I connected a 100 watt light bulb across the fuse holder solder pad points and On. The light blinks for a while and then went off. This was a good sign that there was no major short circuit in the SMPS (please refer to chapter 13) on light bulb test. I then removed the light bulb and put in a new fuse and power on. The SMPS came back to life indicating only fuse problem and no other serious fault in the circuit. Note: This is my normal procedure of troubleshooting any type of SMPS whenever I come across a faulty fuse. A bad fuse is not necessary due to a heavy short circuit. As mentioned, a fuse could go open circuit because of its life span and possibly from a mild surge. Case no 2: The complaint was no power; it was because the fuse had an open circuit. As usual when found that the fuse was not working, I would test on the other major components. The result was the two switching transistors (MJE13007) had shorted and both were mounted on the heatsink (as shown in figure 20.1 in the next page). Further check reveals two fusible link resistors (2.2 Ohm) also had an open circuit.

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Figure 20.1- Power Transistors In ATX Power Supply Please refer to case no 1 about connecting the light bulb before you switch �On� the power supply. Replacement of the 5 components (2 transistors, 2 resistors and 1 fuse) restored the problem. Note: It is quite common when the fuse blown, the bridge rectifier or even the Thermistor and Varistor also will go shorted. Check the Varistor and Thermistor for cracks and pinholes. In some cases, the secondary output diodes might be destroyed too. Case no 3: The complaint was that there was no power but the fuse was working. Since the fuse was good, this shows that there were no major short circuits in the SMPS. Before performing any voltage test, I would usually check on the primary side components like the start up resistor, filter capacitor, non polarized capacitor and even direct replacement of the power IC and retest the SMPS. I will also scan for any short circuit in the secondary side output diodes too. In this case, I found a non polarized capacitor in the primary side with the value of 0.47uf 250 volts dropped to 0.15uf when tested with digital capacitance meter. A replacement solved the no power symptom.

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Figure 20.2- A Faulty Non Polarized Capacitor Can Cause No Power Symptom

Note: - Any components found defective in the primary side could cause a dead power supply symptom. Make sure you test on all the components in the power side and if all components were tested good, then directly replace the power IC (if the power supply used the Mosfet Technology) and retest the power supply. Some ATX power supplies may take you a longer time to troubleshoot because of �too many� components in the board. The most frustrated part was that you could not get the spare parts especially the power IC. Even if you could locate one, the price would be sky high and if this happens, I will usually get the customer to buy a new unit since nowadays the price of a new power supply is quite cheap. 2) Intermittent No Power This ATX power supply came in with the complaint of intermittent no power. When tested it with power on, I could see that the fan was not working. Not only that, some filter capacitors also became bulge because of the heat accumulated inside the power supply. The failure of the fan

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could not suck out the heat generated by the components especially the switch mode power transformer, power transistors and output diodes. If you switched off a working power supply and with the ac power cord removed; and if you touch the power transformer, you could feel that the power transformer was actually quite hot. Another heat source would be the heat sink, where the power transistors and output diodes attached to it. Secondary output Diodes run a little bit hot in filter circuit than diodes in other circuits due to the high current present at the secondary output line.

Figure 20.3- The Right Way To Service The SMPS Fan

As for the malfunctioned fan, you could use a Philips oil based contact cleaner to service the fan. The grease inside the fan would run dry after serving for sometimes and eventually stop rotating. Remove the back plastic cap of the fan and spray it with the contact cleaner and you would be surprised that the fan could regain its glory. It will actually work just like a new fan. If it still doesn�t rotate, the best choice is to replace with a new fan. If you have the power supply at your work place then you could salvage the fan and install it.

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Figure 20.4- Decayed Glue Found In ATX Power Supplies From the photo above, you could see those decayed glue that stick besides the components. Manufacturers purposely use some kind of glue to apply on the components. They think those components will shake or come out when there is vibration or during shipping. The heat generated inside the equipment will turn the glue into conductive after a few years in service and sometimes would even corrode the pins of the components and eventually cause the equipment to have intermittent problem or totally stop working. Scrap the decayed glue off with your test pen and use a Thinner solution to clean the PCB board.

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Figure 20.5- Bad Output Filter Capacitors Could Cause Intermittent No Power Problem

After the replacement of the filter capacitors at the secondary side, with the decayed glue removed and fan serviced, the ATX power supply again was given a new life to faithfully serve the computer.

3) Higher Than Normal Output

Case no 1: When the power supply was switched on, measurements were taken. The results were over voltage. The 12 volts line shot up to 13 + volt and the 5 volts line became 5.6 volts. After the casing was removed, I noticed that the internal part was very dirty and I used a vacuum cleaner and a brush to clean off the dirt. Four filter electrolytic capacitors in the secondary side had bulged. The values of the filter capacitors were 1000uf 10 volt and 2200uf 10 volt.

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Figure 20.6- Dirt Inside The Power Supply

Figure 20.7- After Cleaning With A Vacumm Cleaner And A Brush

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As you know, we as electronic repairers can�t just see things at only one side; we have to see the other side too. What I mean is that try to see if there are any more suspicious components that contribute to the failure of the power supply such as broken components, dry joints, loose connection, decayed glue and etc before the replacement of the four bulged filter capacitors. Some components were covered with decayed glue. I had to carefully remove it by scrapping off the layers of the decayed glue. Once it was done, I cleaned the PCB board with the Thinner solution. As mentioned, decayed glue could cause serious or intermittent problem in electronic equipment because it could be conductive. If you repair any ATX power supply, make sure you check the fan too. Please refer to page 248 on how you can service the fan.

Figure 20.8- The Four Faulty Filter Capacitors Once the four electrolytic capacitors were replaced and the decayed glue removed, the power supply worked like a charm.

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Case no 2: If you found one of the output voltages were higher than normal, say from 5 V to 6 V or from 12 volt to 13 or 14 volt then checked for bad filter capacitors and surrounding circuitry. If all were tested good, do replace the Toroidal inductor as seen in figure 20.9 and retest the power supply. I have personally replaced quite a number of Toroidal inductors that had caused the output voltages to increase.

Figure 20.9- Replace the Toroidal Inductor If All Corresponding

Components Were Tested Good 4) All output voltages were good but with some extra load (connected two additional hard disks) the power supply will shutdown. This problem clearly indicates the secondary filter capacitors have problem because it can�t sustain additional load. True enough, when the casing was opened, I found out that a few of the electrolytic capacitors had bulged. For your information, if the capacitors are good, then you have to test it with ESR meter.

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Figure 20.10- Bulged Electrolytic Capacitors Note: Do not overlook the fact that sometimes bad capacitors in the primary side (be it an non polarized or polarized capacitor) could cause the power supply to shutdown when extra load is connected. 5) High pitched sound from the transformer This power supply was working well except that it produced an annoying high pitched sound. I suspected it was the transformer problem. Before I removed the transformer and dipped it into shellac as explained in page 242, I would usually scan some of the components first. I found two electrolytic capacitors with the value of 1 uf 50 volt had problem (bad ESR) in the primary side and the new parts solved the high pitched sound problem. Note: Before dipping the transformer to the shellac, make sure no other components have problem especially the electrolytic capacitors (check the electrolytic capacitors with ESR meter) otherwise it still would not solve the problem because the problem is in the capacitors and not in the transformer. This will surely waste your precious time. 6) Power transformer has sound and the output voltages very low. Here are the voltages measured at the output: Power good measured 0 volt

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+5 volt measured +1.6 volts +12 volt measured +4.6 volts -12 volt measured -3.6 volt -5 volt measured - 1.4volts This power supply used the half bridge topology where a pair of power transistors (2SC3039) was used to switch the high voltage supplies across the primary winding of the SMPS transformer. Since there were some voltages at the secondary side, this suggests that the primary side was good. Because of this, my concentration was at the secondary side but I found nothing wrong with the secondary output diodes. I then moved on to test the small signal diodes (1N4148) at the input of the small transformer and found that both of the diodes had shorted. By replacing the two signal diodes made the SMPS work again.

Figure 20.11- Low Output Voltages Could Be Caused By Shorted Signal Diodes In The Secondary Side

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Note: Not necessary the secondary output diodes shorted could cause sound in the transformer. It can also be some other shorted components like transistor, PWM IC, and capacitors. Do not overlook that resistors in the secondary side that have an open circuit or turn into high ohm could also cause similar symptom. 7) Output voltages were low and there was no sound from the transformer. The output +5 volt became 2 volt; the 12 volt became 7 volt and power good signal was 0 volt. The fan could rotate (quite slowly but did not stop) and once the power supply was connected with extra load (hard disk) the fan stopped and power shut down. This problem normally was the cause of bad secondary filter capacitors but somehow all the capacitors were tested okay. Since there were voltages at the output, this suggests that the switching transistors were good. It could also be some other component failure in the primary side that could affect the �on� time of the switching transistor thus producing a lower than normal output. Usually, capacitors in the primary side tend to have problems, so I checked on the non polarized capacitor first. Guess what? The non polarized capacitor .47uf 250v capacitance value became .19uf causing the SMPS to produce a lower output. Please refer to figure 20.2 to see where the non polarized capacitor is located. 8) No power good signal but all other voltages was good Normally, when there is no power good signal (gray wire), we will trace back from the wire and check to see which location the signal was lost or to see if the signal source has any problem or not. But in this case, it seems that all the secondary side section was okay. I did not give up and try to check the primary side just in case if there is problem in the components. I knew the chances were quite slim because the SMPS has all the output (which means the primary side is working) except the power good signal only. I went ahead and test the big filter capacitor and surprisingly one of the big filter capacitors has problem. The value is 470uf 200 Volt and a replacement brought back the 5 volt power good signal.

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Figure 20.12- One Of The Filter Capacitor Problem Could Cause No Power Good Signal

Note: Sometimes when you think that the problem lies in the secondary side it could also have some percentage of failure in the primary side. If you have exhausted finding fault in the secondary side, then try the primary side and vice versa. In the above case, I believed the ripples which were not removed by the big filter capacitor had entered into the secondary side causing problem to the power good signal. 9) Fan rotates a bit and then stopped and the transformer has sounds. Case no 1: As usual, whenever I come across this problem I will check on the secondary side filter capacitor with ESR meter and the secondary output diodes. Since the filter capacitors were tested good, I then proceed to check on the secondary output diodes. I will remove the big SMPS transformer and then check all the secondary diodes. The reason I removed the SMPS transformer was that I could test all of the secondary diodes (ultra fast recovery diode and Schottky diodes) on board without removing one of the diode leg. Secondly, sometimes I do come across decayed glue located underneath the transformer output pins and it will cause intermittent problem and you could hardly see the decayed glue from the top.

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Checking all the diodes this way will give you a more accurate reading as compare when you check the diodes on board. Off course you can remove one lead in all the secondary diodes and test it with your meter-it�s your preference. In this case, I found a diode shorted in the -12 volt output line (blue wire) and a new diode cured the problem.

Figure 20.13- A Shorted Diode In The Secondary Side Could Cause Fan To Turn A Bit And Stop

Note: Please do not think that every time when there is a complaint regarding the SMPS fan turning a bit and then stop, it must be the cause of a shorted secondary diode or bad filter capacitor. It could be due to other causes too- please read the next case to see what the solution to the problem is. Case 2: First I thought it was one of the secondary output diode that was problematic but I found all of them were good. The secondary output filter capacitors were tested good too and the PWM TL494CN IC was replaced with no improvement. My concentration now was to check on the corresponding components of the PWM TL494CN IC. All components with one leg were lifted up and tested with meter and found two resistors had problem. A 47k Ohm that was connected to pin 14 of the PWM TL494CN IC changed to 70 over K Ohm and another resistor

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4.9 K Ohm that was connected to pin 1 of the IC had an open circuit. By replacing these two resistors brought the power supply back to life.

Figure 20.14- Faulty Corresponding Components Could Cause Fan To Rotate A Bit And Stop.

Note- It is not necessary whenever the fan rotates a bit and then stops, it must be the cause of shorted secondary output diodes. From the solution given above; it turned out to be the bad components that were located surrounding the PWM TL494CN IC. The PWM IC could turn out to be any part number and it depends on what model and type of power supply you are working on. A shorted PWM IC could also cause similar symptom too.

Is It Worth To Repair Computer ATX Power Supplies?

Many questions arise whether one should continue to repair Computer Power Supply or not since a new unit is quite cheap nowadays. As for me, I had stopped repairing ATX power supply as the cost of a new unit is very cheap. It�s not worth repairing because the spare parts sometimes are much more expensive than getting a new power supply. Searching for ATX power supply spare parts was not easy as many of them couldn�t be found from the Internet as not only that as many complicated and

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different designed by power supply manufacturers had used up our precious troubleshooting time. We need time to understand how all these different designed power supply works. As manufacturers want the design to be made into compact size, many secondary or even primary power supply circuit are built into a modular board (smaller board). This made troubleshooting even more difficult because many times the meter�s probe can�t reach the testing point. The real reason why I had stopped repairing ATX power supply was the profit margin. If you charged too high, the customers would rather buy a new unit that comes with a one year warranty. If you charged too low, you may end up in the losing side because of the components replaced, electricity and etc. If you charge a reasonable fee, the profit margin gained cannot even cover your time spent on troubleshooting it. I�m here not to discourage you to stop repairing ATX power supply. However, if you have the time, contacts of getting cheap power supply components, easy to access many power supply schematic diagrams and etc then you may go ahead to repair it. If your reason of repairing power supplies is to improve yourself with troubleshooting skill and experience and not for the profit purposes then you are encourage to repair the power supplies. Conclusion- There are many designs of Computer ATX power supply in the market, thus you must be flexible on how to troubleshoot them. The true case histories were just a guide to show you on how I tested, troubleshoot and repaired the power supplies. You may encounter power supplies with the same problem as in the true case histories but you may also encounter new problem that you haven�t seen before. No matter what the problem is, just reread my book again and follow the procedure and use your imagination on how to access to the bottom side of the printed circuit board. You have to do it carefully because twisting and turning the PCB board too many times can cause attached wires (the AC or the output wires) to break loose. I�ve fixed thousands of switch mode power supplies over the years and from experience, I could say that the more you work on power supplies, the better you are. I also believe that, the more time you spend on reading this book you will definitely be able to solve many problems in Computer ATX SMPS. Keep the safety precautions in mind and make sure the filter capacitors are discharged and you should be safe.

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Part V

Miscellaneous

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21) Understanding Power Supply Glossary Term

AC (Alternating Current) � A current that periodically reverses its direction of flow. The electricity supply to homes, offices, factories and etc. is AC. AC Line � A power line that delivers alternating current only. AC Line Filter � A filter designed to remove extraneous signals or electrical noise from an AC power line, while causing virtually no reduction of the power line voltage or power. AC Line Voltage � The voltage commonly delivered by the commercial power line to consumers. In the United States, the two standards are 117V and 234V (~ about 5 percent). The lower voltage is used by most appliances; the higher voltage is intended for appliances and equipment that draws high power, such as electric ovens, cooking ranges, clothes dryers, and amateurs-radio amplifiers. In Europe, 220V is the common standard. AC Noise � Electromagnetic interference originating in the AC power lines or electrical noise of a rapidly alternating or pulsating nature. Active Component- A device capable of some dynamic function (such as amplification, oscillation, or signal control) that usually requires a power supply for its operation. For examples: bipolar transistors, field effect transistors (FET) and integrated circuit (IC). Attenuates- To reduce in Amplitude Bleeder- A resistor or group of resistors, used permanently to drain current from charged capacitors. It establishes the predetermined initial load level for a power supply or signal source, and it serves a safety device in high voltage power supplies. Crest Factor - This is the ratio between the peak current and the average current required by the load. Computers normally exhibit a crest factor of 2 to 3, which means the computer draws two to three times the average current for short duration, such as the starting time etc.

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Current - It is a quantitative measure of the amount of electricity passing through a circuit or the movement of charge carriers, such as electrons, holes, or ions. The unit for measuring the current is Amperes. Current Limiting � The controlling of current so that it does not exceed a desired value. Current Limiting Resistor- A series resistor inserted into a circuit to limit the current to a desired value. Current Meter � A normally direct-reading instrument, such as an ammeter, milli ammeter, or micrometer, used to measure current strength. Current Noise � Electrical noise produced by current flowing through a resistor. DC (Direct Current) - It has either a positive or negative polarity and flows in one direction. A DC charge can be stored more easily and is used for all batteries. DC Power Supply � A power unit that supplies direct current only. Examples: battery, transformer / rectifier / filter circuit, DC generator, and photovoltaic cell. DC Voltage- A voltage that does not change in polarity, an example being the voltage delivered by a battery or dc generator. DC Working Voltage- The rated dc voltage at which a component can be operated continuously with safety and reliability. EMI- Any electronic device generates electromagnetic waves. If such electronicmagnetic waves interfere with another device through spatial radiation or a power cord, it is called EMI or Electromagnetic Interference. ESR- ESR stands for Equivalent Series Resistance and is an effective resistance that is used to describe the resistive parts of the impedance of certain electrical components. Impedance - It is combination of resistance, inductance and capacitance which restricts the current through any device. Inverter - It is a circuit which converts DC to AC.

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Line Frequency - It is the number of times the AC flows in one direction during one second. Frequency is measured in Hertz (Hz) or cycles per second. The standard power frequency may differ. For example the AC frequency in Malaysia is 50Hz. (i.e. the current changes its direction 50 times per second) while in the U.S.A. it is 60 Hz. Line Loss- The sum of energy losses in a transmission line. Line Noise- Electrical noise (as received by a radio) arising from fluctuations of current or voltage in a power line. Passive Component- A device that is basically static in operation (it is ordinarily incapable of amplification or oscillation and usually requires no power for its characteristic operation). For example: resistor, capacitor, diode inductor, fuse and rectifier. Power Factor - This is the ratio of real power to apparent power (VA/watts). The power factor can be �leading� or �lagging� depending upon the type of load. Inductive loads cause the current to lag and capacitive loads causes the current to lead the voltage. Power Factor Meter � An instrument that gives direct readings of power factor (lead or lag). One such meter uses a dynamometer-type movement in which the rotating element consists of two coils fastened together at right angles. Power Line Frequency- The frequency of the alternating current and voltage available over commercial power line. In the United States, the power frequency is 60 hz while some countries is 50 hz. Power Rating � The specified power required by equipment for normal operation. Power Supply � A device, such as a generator or a transformer-rectifier-filter arrangement, which produces the power needed to operate on electronic equipment. Power Surge- A momentary increase in the voltage on a utility line. Power Switch- The switch for controlling power to a piece of equipment. Power Transistor � A heavy-duty transistor designed for power-amplifier and power-control service.

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PWM - Pulse Width Modulation is a technique employed to regulate the output power by changing the pulse width. PWM is employed is SMPS, UPS and many other power control applications. Rectifier - It is a diode network which changes AC to DC. The process of changing the AC to DC is call rectification. This is reverse of an inverter which converts DC to AC. Ringing- Self oscillation in a pulsed inductance-capacitance circuit, sustained by the circuit�s flywheel action, and usually producing a damped wave. Ripple- A small alternating current component in the output of a direct current power supply with inadequate filtering. Sine Wave - It is the most simple of all wave forms and is the shape of the AC in homes, offices etc. In a sine wave, the voltage or current changes smoothly from a negative maximum to a positive maximum with changing time. Square Wave - This waveform is similar to the sine wave except that the transitions from negative maximum to positive maximum are abrupt. Most domestic inverter�s supply a square wave as it is simple to generate. Surge Suppressor. A semiconductor device used to absorb potentially destructive transients or over voltages on a utility power line. Transformer - A device used to convert an AC voltage to different AC voltage levels. A transformer is also used to isolate the output AC power from the source. Transient- A sudden high voltage spike in an alternating current system, caused by arcing or lightning VA (Volt Ampere) - This is the simple product of voltage and current and is used to express the amount of power. VA gives the apparent power. Voltage - It is a measure of the potential difference between two points, it is what causes the current to flow from a higher potential. This is measured in volts. Voltage Breakdown-The voltage at which current suddenly passes in destructive amounts of dielectric.

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Wattage - This is a measure of energy drawn per second by the load. It is calculated by multiplying the VA by the power factor (VA x pf). This gives the true power. The pf varies between 0 for ideal inductive and capacitive loads to 1 for pure resistive loads (incandescent lamps). The pf for a typical computer would be between 0.6 and 0.8 in most cases. Wattmeter- An instrument used to measure electrical power. The scale usually reads directly in watts, kilowatts, milliwatts, or microwatts. Wave Form - This is the name given to the shape followed by any alternating current or voltage. Winding- A coil in an inductor or transformer such as the primary and secondary winding.

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22) Recommended Electronic Repair Ebook

Book Title: LCD Monitor Repair Author : Jestine Yong Pages: 200 Book format: EBook Price: USD47.77 Website: www.LCD-Monitor-Repair.com About The Book: A step by step guide on how you can become a Professional in LCD Monitor Repair. It teach you how you can solve LCD Monitor problems like no power, display dim, display shutdown and many more. Book Title: Testing Electronic Components Author : Jestine Yong Pages: 166 Book format: EBook Price: USD37.00 Website: www.TestingElectronicComponents.com About The Book: A step by step guide on how you can test electronic components like a professional. It covers electronics components from resistors, capacitors, transistors up till switch mode power transformer. If you know how to test electronic components, you can start electronic repair work.

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Book Title: How To Find Burnt Resistor Value Even Without A Schematic Diagram Author : Jestine Yong Pages: 50 Book format: EBook Price: USD24.00 Website: www.FindBurntResistorValue.com About The Book: How you can find the burnt resistor value in electronic circuit even without a schematic diagram. It comes with diagrams and true case histories to easily help you to locate the value of a burnt resistor.

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23) Recommended Resources

1. Power Supply Troubleshooting & Repair By Lanny L.Logan

2. Troubleshooting & Repairing Colour Television Systems By Robert L. Goodman.

3. Troubleshooting & Repairing Solid-States TVs By Homer L

Davidson

4. Samuel M. Goldwasser �Notes On The Troubleshooting And Repair Of Small Switch Mode Power Supplies�. Here is the website link http://www.repairfaq.org/sam/smpsfaq.htm

5. List of Electronic Spare Parts Suppliers at JestineYong dot com.

Here is the website link http://www.jestineyong.com/?cat=12

6. ATX Power Supply Repair website- http://www.smps.us/computer-power-supply.html

Free Electronic Equipment Schematic Diagrams

1) www.EserviceInfo.com 2) www.Protech2u.com 3) www.FastRepairGuide.com

Buy Electronic Equipment Schematic Diagrams 1) www.Radiolocman.com 2) www.justmanuals.com 3) www.servicemanuals.net

Electronic Repair Forum 1) http://forum.eserviceinfo.com 2) www.Repairworld.com

Electronic Repair Website

1) www.ElectronicRepairGuide.com 2) www.Anatekcorp.com 3) www.Epanorama.net/links/repair.html

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Electronic Repair Membership Websites

1) www.ElectronicRepairGuide.com/Recommend/PlasmaTelevisionRepair.htm

2) www.ElectronicRepairGuide.com/Recommend/LCDTelevisionRepair.htm

3) www.ElectronicRepairGuide.com/Recommend/ProjectionTelevisionRepair.htm

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24) Conclusion I strongly suggest you to reread this information for few times and start right away all of the tips and tricks you have learned from this E-book. If you have questions about Power Supply Repairs or even in electronic repair, please do not hesitate to email me at jestineyong@electronicrepairguide.com I wish you all the best and look forward to hearing your success story. To your success, Jestine Yong Bsc. Eng UK Author of �Troubleshooting & Repairing Switch Mode Power Supplies� http://www.lcd-monitor-repair.com http://www.testingelectroniccomponents.com http://www.findburntresistorvalue.com http://www.electronicrepairguide.com http://www.jestineyong.com http://www.noahtec.com